1. Field of the Invention
This invention relates generally to diamond cutting elements employed in drag bits for drilling subterranean formations, and more specifically for a structure and method for cooling the diamond tables of such cutting elements and their associated substrates
2. State of the Art
Diamond cutting elements have been employed in earth boring drill bits for many decades. During the last twenty years, synthetic diamonds, known as polycrystalline diamond compacts, or PDC's, have been made available on the market by General Electric, DeBeers and others. These PDC's are available in a variety of shapes, but one preferred configuration widely employed in earth boring drill bits is a planar disc, variations of which, including half-discs and "tombstone" shaped planar cutters, have also been employed Non-planar diamond cutting elements have also been developed and proposed, such as "dome" cutters and concave cutters. Advances in diamond film technology enhance the feasibility of non-planar cutters of fairly complex configuration.
Planar PDC cutting elements generally comprise an assembly of a layer of diamond crystals bonded together under ultra-high temperature and pressure into a wafer-like, thin layer or "diamond table" on a cemented carbide (such as WC) substrate of similar configurations. The PDC cutting element is then bonded via the substrate (as by brazing) to a carrier element such as a stud, cylinder or other supporting structure, which in turn is secured to the face of the drill bit. It will be appreciated that some PDC cutting elements may not possess a uniform thickness diamond table and, as with overall cutter configuration, diamond film technology presents many potential options for varying diamond table thickness.
PDC's have been a great success in advancing the state of the drill bit art, and are now widely employed in drill bits of numerous and diverse configurations. Since the early days of PDC use on drill bits, however, it has been apparent that PDC's suffer thermal degradation at the high temperatures generated by the frictional abrasive contact of the PDC cutting edge with the formation as the bit rotates and weight is applied to the drill string on which the bit is mounted. Such degradation leads to premature dulling of the PDC cutting edge, and even gross failure of the PDC cutting element assembly. Improved feed stock and fabrication techniques have raised the thermal tolerance of PDC's to some degree, and there has developed a subcategory of PDC's known as thermally stable products, or TSP's, which retain their physical integrity to temperatures approaching 1000.degree. C. TSP's may be infiltrated into matrix body drill bits at the time of bit furnacing, rather than being attached at a later time, as with non-thermally stable PDC's. However, even TSP's suffer from thermal degradation during cutting of the formation as the drill bit advances the well bore.
While the prior art has focused on problems associated with the degradation of the diamond layer or table, heating of the cutting element substrate (typically tungsten carbide) from the drilling operation is also detrimental to cutting element performance. Heat checking of the substrate, typically caused by alternative heating and quenching of the cutting elements as the drill bit bounces on the bottom of the borehole, can initiate more severe substrate cracking which, in turn, can propagate cracking of the diamond table.
A variety of attempts have been made to cool and clean PDC cutting elements during the drill operation by flushing the cutting elements with drilling fluid, or "mud," pumped down the drill string and through nozzles or other orifices on the face of the bit. The flow of drilling mud removes formation cuttings and other debris from the face of the bit and generally radially outwardly to the bit gage, up the junk slots and into the well bore annulus between the drill string and the wall of the well bore to the surface, where the debris is removed, the mud reconditioned with additives and again pumped down the drill string. It is known in the art to direct drilling mud flow across the face of a series of cutting elements (U.S. Pat. No. 4,452,324 to Jurgens); to direct mud flow from a nozzle toward the face of a single cutting element (U.S. Pat. No. 4,303,136 to Ball); and to direct flow from a nozzle to a single cutting element at a specific orientation (U.S. Pat. No. 4,913,244 to Trujillo). It has also been proposed to direct mud flow through the face of a PDC cutting element from internal passage extending from the interior of the drill bit through the carrier element and out an aperture in the face of the cutting element (U.S. Pat. No. 4,606,418 to Thompson). All of the foregoing approaches, while providing some cooling to the PDC cutting element, are believed to serve primarily to remove formation cuttings from the cutting elements, and only incidentally or secondarily to provide any benefit in cooling the cutting element, as the mud flow is actually quite removed from the high temperature point or line of contact between the outermost edge of the PDC cutting element (taken from the bit face) and the uncut formation. Stated another way, the intervening presence of the formation cuttings or chips being sheared from the formation at the PDC cutting edge prevents contact between the drilling mud flow and the high temperature interface between the cutting element and the formation in the vicinity of the cutting edge.
It has been proposed, in U.S. Pat. No. 4,852,671 to Southland, to direct drilling mud flow through a passage in a stud supporting a PDC to a relief between the pair of cutting points in the formation-contacting zone of a disc-shaped PDC cutting element to improve the cooling and cleaning of the cutting elements. While potentially an improvement over the previously-referenced externally-disposed drilling mud flow direction techniques, the Southland patent suffers from the limitations imposed by the use of a single fluid exit point proximate the cutting contact point of the PDC. Moreover, flow characteristics of the Southland cutter will commence an almost immediate deterioration as soon as drilling commences, it being generally known that unused planar PDC's, even those with so-called "chisel" configurations, or those of the Southland "double point" configuration, wear rapidly during the first part of a drilling operation. Thus, after a few dozen feet of drilling, the Southland cutter points are worn flat to the depth of the relief, and the fluid intended to flush and cool the cutting zone is ejected from the stud passage behind the cutting element, to no advantageous effect.